I have a lot of family members who own rental units or apartment or multiple condo units.
You will need A LOT of extra time in the front end to make sure the units are suitable for rent. Meaning working appliances and clean and freshly painted and newer flooring. That's for lower end unit. Then if they are high end units and you want top dollar you'll need nice upgrades like granite counters, etc.
THE MOST IMPORTANT decision you have to make, that you must not waiver on, is you MUST rent to only people with good credit. NO SOB stories. You'll be doomed if you ever once let your heart get the best of you. You must do your research and check every applicant's rental history to ensure they were decent citizens who left their last place in a orderly fashion, without neighborly complaints, etc. And you must check employment and at least 3 references.
If you don't, be forewarned, the general public who does not pass these tests will destroy your units.